The cornea is one of the most densely innervated tissues in the body. Corneal nerves originate chiefly from the ophthalmic branch of the trigeminal ganglion (TGG) and are largely sensory, responding to noxious mechanical, thermal and chemical stimuli by transducing sensations of pain (nociceptors). They are also involved in protecting the cornea from damage by modulating the blinking reaction and increasing the production of tears, and they are required for maintaining the cornea in a healthy state. Thus, pathological conditions that interfere with normal innervation and nerve function can have deleterious consequences, ranging from chronic pain to ulceration and loss of transparency. However, little is known concerning the mechanisms responsible for corneal innervation. The embryonic chicken provides an advantageous developmental model for investigating corneal innervation, as in this species innervation occurs in a series of discrete stages that are temporally and spatially separable from one another. These stages are: (1) the growth and attraction of nerves from the TGG to the cornea, (2) invasion of nerves into the corneal stroma and (3) the formation of branches from the nerves that enter and innervate the corneal epithelium (CE) where they terminate. In the studies to be proposed one of the goals will be to elucidate the mechanism(s) involved in regulating these events. The other area of investigation will be to examine the fate of these nerves once they enter the CE - as this is where sensation occurs. The conventional belief is that these nerves terminate as free nerve endings. However we will examine an alternative, which is that they interact with CE cells in a manner that is structurally and functionally unique to the cornea. PUBLIC HEALTH RELEVANCE The cornea is one of the most densely innervated tissues in the body. These nerves are largely sensory, responding to noxious mechanical, thermal and chemical stimuli by transducing these as sensations of pain. The corneal nerves are also involved in protecting the cornea from damage, by modulating the blinking reaction and increasing the production of tears, and they are also involved in maintaining the cornea in a healthy state. Thus, pathological conditions that interfere with normal innervation and nerve function can have deleterious consequences - ranging from chronic pain , to ulceration, to decreased transparency. For example, a disruption in corneal innervation - as can occur by damage from herpes infection - can result in degenerative neurotrophic keratitis that can produce corneal ulceration and a loss of transparency. Also, following vision corrective surgical procedures (e.g., PRK and LASIK), a decrease in innervation can occur that can lead to post-surgical complications. However, surprisingly little is known concerning the mechanisms responsible for corneal innervation, including those that are involved in embryonic development. For developmental studies of corneal innervation, the embryonic chicken cornea provides an advantageous model which will be used for the proposed studies. In most aspects the chicken cornea is indistinguishable from that of the human. Structurally this cornea has all the layers of the human cornea, and the molecular compositions of these layers are essentially identical to those of the human. In addition, in the embryonic chicken cornea, innervation occurs as a series of discrete stages, each which can be studied separately. Elucidating the cellular and molecular mechanisms that are involved in these different stages of innervation is one of the major goals of the proposed studies. The other major area of investigation will involve how, within the cornea, sensation is transferred to the nerves. The conventional belief is that this involves nerves that terminate within the corneal epithelium as free nerve endings. However, based on recent observation in our laboratory, we will examine an alternative, which is that the endings of the corneal nerves interact with specialized corneal epithelial cells to produce a sensory unit that is structurally and functionally unique to the cornea.